Most of the existing lubricating oil compositions, such as hydraulic oils for construction equipment, grease oils, chain saw oils used for tree cutting, and two cycle engine oils for leisure boats, contain mineral oils as a main component. In the above uses, scattering and leakage of oil cannot be avoided, resulting in contamination of rivers, ground-water, soil, the ocean, etc. Since the biodegradability of conventional lubricating oil compositions containing mineral oils as a main component are low, contamination of environment with such conventional lubricating oil compositions has recently become a more and more serious problem.
In order to solve the above problem of environmental pollution, there have been reports on lubricating oil compositions of which biodegradability is improved by using natural fats and oils, such as rapeseed oil, as a main component. For example, Japanese Patent Laid-Open No. 5-230490 discloses a biodegradable chain oil comprising 80 to 98% by weight vegetable oils, such as rapeseed oil, soybean oil, sesame oil and castor oil, and 2 to 20% by weight additives. Japanese Patent Unexamined No. 5-503949 discloses a hydraulic oil prepared by blending purified rapeseed oil and/or soybean oil as main components with specific oxidation inhibitors and ester components.
However, lubricating oil compositions containing natural fats and oils as a main component present a problem of thermal oxidation instability because natural fats and oils have many unsaturated bonds and are liable to oxidative degradation.
WO95/02659 discloses a highly biodegradable base oil for hydraulic oil, the main component of which being a compound prepared by esterifying an addition product of 0.5 to 3 mol of ethylene oxide and/or propylene oxide to 1 mol of glycerol with a saturated or unsaturated fatty acid having 6 to 24 carbon atoms, or a compound prepared by intramolecularly adding 0.5 to 3 mol of ethylene oxide and/or propylene oxide to 1 mol of natural fats and oils. However, when the amount of ethylene oxide and/or propylene oxide added to 1 mol of glycerol is in the range of from 0.5 to 3 mol, it is necessary to increase the number of unsaturated bonds in the resulting compound to meet the fluidity requirements of hydraulic oils at low temperatures. Thus, the compound of this prior art does not solve the problem with respect to thermal oxidation stability to be improved in the present invention.
Japanese Patent Laid-Open No. 1-230697 discloses a metal working lubricant which comprises, as an essential component, an addition product of an alkylene oxide to a mixture containing natural fats and oils and a trihydric or higher polyhydric alcohol, in which 10 to 100 mol of alkylene oxide is added to 1 mol of natural fats and oils. The object of the lubricant of this prior art is to eliminate the drawbacks of emulsion-type lubricants in the field of metalworking technology by making natural fats and oils of triglyceride structure soluble in water without impairing their lubricity. Therefore, there is no mention about biodegradability of the lubricant. Unlike the above-mentioned hydraulic oil, chain saw oil and two cycle engine oil, this prior art lubricant is used in a setting where its biodegradability has no significance in terms of environmental protection.
As lubricating oils for refrigerating machines using fluorocarbon as a refrigerant, Japanese Patent Laid-Open No. 4-328197 discloses a lubricating oil for fluorocarbon refrigerant having, as a main component, a compound obtained by esterifying the terminal hydroxyl groups of a glycerol type polyalkylene polyol with an aliphatic monocarboxyl compound. In addition, Japanese Patent Laid-Open No. 2-276881 discloses a composition for refrigerating machines using tetrafluoroethane refrigerant, the composition being obtained by acylating all or part of the terminal hydroxyl groups of a polyalkylene polyether of monohydric, dihydric or trihydric alcohols.
However, the above compounds used as lubricating oils for refrigerating machines have been developed focusing on the compatibility with fluorocarbon refrigerants, because the compounds are used in the atmosphere of fluorocarbon.
In order to prevent environmental pollution, hydraulic oils and grease oils, which are used in a setting where their leakage into natural environment cannot be avoided, are required to be highly biodegradable as well as highly stable to thermal oxidation. Hydraulic oils and grease oils which meet these requirements have yet to be known.